This application is directed to high definition television (HDTV) and particularly to an improvement in high definition television which is compatible with standard NTSC (National Television System Committee) television.
There is a growing interest in the broadcasting field in high definition television because of its ability to deliver a video signal which produces improved resolution compared to standard NTSC television which has been in use for years. In general, in high definition television, more picture elements are displayed on a screen.
In the NTSC system, vertical resolution is limited by the number of scan lines. In standard NTSC television there are 525 scan lines, 483 of which typically contain active video. Thus, along the vertical axis, information must be displayed at discrete sites. In contrast, horizontal resolution is limited by bandwidth. Pixels may be positioned anywhere continuously along the horizontal axis, so that horizontal resolution is only a measure of how close two resolvable pixels on the same line can be positioned with respect to each other. For the 4.2 MHz luminance bandwidth specified for the NTSC system, 440 lines are achievable for the full screen width.
As described in "High Definition NTSC Broadcast Protocol", IEEE TRANSACTIONS ON BROADCASTING, Vol. BC-33, No. 4, pages 161-169, December 1987, by Richard J. Iredale, the contents of which are hereby incorporated by reference, there have been a number of different types of systems proposed for high definition television. For example, the system described in "An HDTV Broadcasting System" Utilizing a Bandwidth Compression Technique-MUSE, IEEE TRANSACTIONS ON BROADCASTING, Vol. BC-33, No. 4, December 1987, pages 130-160, by Ninomiya et al. involves a direct broadcast satellite having a satellite channel with a bandwidth of 27 or 24 MHz. While the quality of the picture delivered by this system is apparently very good, as explained in the Iredale publication, such a system is incompatible with the numerous NTSC home television receivers currently in use. That is, use of a system such as that described in the Ninomiya et al. publication would render existing home television receivers incapable of receiving the high definition television broadcast.
In view of the large number of NTSC home television receivers currently in use, it has been proposed that a high definition broadcast system should be compatible with NTSC video, so that NTSC home television receivers will be able to receive programming transmitted by a high definition television broadcast system.
The Iredale publication discloses an NTSC-related multiple subsampling technique for transmitting high definition pictures in a conventional NTSC channel while maintaining compatibility with existing NTSC television receivers. Subsampling is performed by using a sample and hold circuit to sample a video signal and produce a sample and hold waveform. If the video signal has been sampled at the Nyquist rate or higher, then the sample and hold waveform can be passed through a low pass filter to reconstruct the original waveform. Subsampling has been used in high definition television to achieve increased spatial resolution at the expense of temporal resolution. Thus, while standard NTSC television produces approximately 30 frames per second on the screen, a subsampled high definition television system will produce a lower number of frames per second but with a greater number of pixels in the picture. This can be a problem if an object in the picture is moving, and may make it necessary to reduce the spatial resolution of moving objects in the high definition picture in order to adequately display motion. However, the greater spatial resolution of a non-moving picture through use of an HDTV system is clear.
Of the NTSC compatible high definition television systems which have been proposed to date, images viewed on an NTSC receiver have been subject to scene dependent flicker. This flicker results from frame-to-frame differences in the NTSC signal caused by the interleaved nature of the subsampling. The effect is greatest near edges and in other high detail areas of the picture. The cause for such flicker is explained with reference to FIG. 1 which is a diagram showing the mapping of a high definition television picture in which plural subpixels a, b and c are mapped onto a single pixel in the NTSC picture. The subsampling employed in the high definition television picture illustrated by FIG. 1 is directed to horizontal subsampling to achieve horizontal resolution which is a multiple of the NTSC resolution. In NTSC frame 1, subpixel a from position L.sub.1 in the HDTV picture is mapped onto pixel x which is at location L.sub.1 ' in the NTSC picture. In successive NTSC frames, subpixels b and c are also mapped onto pixel x at position L.sub.1 ' of the NTSC picture. Thus, in successive frames, the three spaced apart subpixels a, b and c from the HDTV picture are mapped onto a single geographic location L.sub.1 ' in the NTSC picture. Flicker is produced because of the difference in brightness values in the subpixels a, b and c which are scanned onto the same position in the picture. Since use of the above-described high definition video signal will degrade the picture on an NTSC receiver by producing this flicker which is not present when a standard NTSC video signal is employed, there is a desire to utilize techniques to minimize this flicker effect in the interest of compatibility.
It should be noted that it has also been suggested to perform vertical subsampling in order to double the scanning lines in the HDTV picture.
If alternate adjacent scan lines from the HDTV picture are used in alternate frames of the NTSC picture, vertically adjacent pixels will be mapped into a common location on the NTSC screen in a manner which is analogous to the horizontal subsampling arrangement described above with respect to FIG. 1. As a result, vertical subsampling will also produce flicker, and particularly edge flicker.
There is a need in the art for a system for converting a high definition video signal into a subsampled video signal compatible with NTSC transmission. Further, there is a need for such a system which is capable of being used for vertical and horizontal subsampling. There is a further need in the art for a circuit which can be provided in an advanced or high definition receiver to reconstruct the high definition video signal based on the subsampled video signal which is compatible with NTSC transmission.